As use of portable electronic apparatuses rapidly spreads, the specifications required for a battery used with these apparatuses have become increasingly stringent over the years. In particular, such a battery has been required to be small and thin and have a large capacity, an excellent cycle characteristic, and stable performance. In the secondary battery field, attention has been paid to nickel hydrogen batteries and lithium non-aqueous electrolyte batteries for their higher energy density than other kinds of batteries. The share of the both types in the secondary battery market is substantially growing.
Many apparatuses using these types of secondary batteries have a prismatic (flat box-shaped) space for housing a battery. Therefore, sealed secondary batteries that have a power generating element housed and sealed in a prismatic outer can have often been used for such apparatuses. An example of such prismatic sealed secondary batteries will be described with reference to one of the accompanying drawings.
FIG. 5 is a perspective view showing a cross section in a vertical direction of a related-art lithium non-aqueous electrolyte secondary battery which is a prismatic sealed secondary battery. In this sealed secondary battery 10, a flat scrolled electrode 14 in which a positive plate 12 and a negative plate 11 are wound with a separator 13 therebetween is housed in a prismatic battery outer can 15. The outer can 15 is sealed by a sealing plate 16.
The positive plate 12 of the flat scrolled electrode 14 is wound so as to be positioned at the outmost edge of the electrode and thus exposed. The positive plate 12, positioned at the outmost edge and exposed, comes into contact with and is electrically connected to an inner face of the prismatic battery outer can 15, which also serves as a positive terminal. The negative plate 11 is electrically connected to a negative terminal 18, mounted at the center of the sealing plate 16 with an insulator 17 therebetween, via a current collector 19.
Since the prismatic battery outer can 15 is electrically connected to the positive plate 12, it is necessary to prevent a short-circuit from occurring between the negative plate 11 and the prismatic outer can 15. For that purpose, an insulation spacer 20 is inserted between an upper end of the prismatic scrolled electrode 14 and the sealing plate 16 to establish insulation between the negative plate 11 and the prismatic outer can 15.
The prismatic non-aqueous electrolyte secondary battery 10 is manufactured by inserting the flat scrolled electrode 14 inside the prismatic battery outer can 15, then laser welding the sealing plate 16 to the opening of the battery outer can 15, and injecting non-aqueous electrolyte from an electrolyte injection hole 21 to seal the electrolyte hole 21. This method for fixing the sealing plate 16 to the prismatic outer can 15 by laser welding as described above has widely been used for its advantageous effect of tightly sealing the opening of the prismatic outer can 15.
The method for manufacturing the sealed battery by laser welding the sealing plate to the prismatic outer can as described above has an excellent advantage of allowing a reduction in weight of the prismatic battery, in particular, if aluminum or aluminum alloy having excellent thermal conductivity is used as the material for the prismatic outer can and the sealing plate. However, a crack tends to occur in a weld between the sealing plate and the prismatic outer can, which may significantly reduce the yield of the product. The reason for the tendency of a crack to occur in the weld between the sealing plate and the prismatic outer can will be described with reference to FIG. 6. FIG. 6 is a sectional view showing a related-art method for laser welding a sealing plate to an outer can of a prismatic battery as disclosed in JP-8-77983-A (claims, paragraphs [0018] to [0022], FIGS. 2 to 4). Elements of a prismatic sealed battery as shown in FIG. 6 are given the same reference numerals as those of the abovementioned prismatic sealed battery.
The sealing plate 16 is set inside the outer can 15 of the prismatic battery and then a laser beam is radiated on the boundary therebetween. Consequently, a metal material such as aluminum in areas indicated by the chain lines is heated up to about 1,000° C., melted and welded. When aluminum is used as the material for the prismatic outer can 15 and the sealing plate 16, the depth of the melted portion indicated by the chain lines is about 0.2 to 0.3 mm. In the heated and melted metal, heat is conducted in the directions indicated by the arrows, whereby the metal is cooled and hardened. While the heat conducted in the arrow directions is radiated from surfaces of the prismatic outer can 15 and the sealing plate 16, the heat is more efficiently radiated from the corners of the prismatic outer can 15, reducing the temperature. In general, heat is efficiently conducted to a lower temperature portion, so the heat in the melted portion is more efficiently conducted in a direction indicated by the arrow A. Consequently, the melted portion is cooled from an outer part thereof and hardened in the order of a, b, and c regions. In other words, the hardened region expands from outside to inside as indicated by the arrow B.
The volume of metals shrinks when cooled, thus hardening the metal. Therefore, the melted portion shrinks in volume when its outer part is hardened, and thus a part of the interior of the melted portion, which is still being melted, moves to outside. Then the interior of the melted portion shrinks in volume when hardened. Further, the part of the interior of the melted portion that has moved outward causes tensile stress. This causes a crack in a boundary between the prismatic outer can 15 and the sealing plate 16, which has a low tension strength. Such a crack is more likely to occur in the corners of the prismatic outer can 15 due to efficient heat radiation from the surfaces of the corners. This kind of crack occurs not only when the prismatic outer can and the sealing plate are laser welded but also when a circular outer can and a sealing plate are laser welded or when an electron beam is used for welding instead of a laser beam.
In the invention of the method for manufacturing a prismatic sealed battery as disclosed in JP-8-77983-A, an edge of the prismatic outer can 15 is cut off by a predetermined angle α to form a heat radiation eliminating portion 15′, as shown in FIG. 7. This is to reduce the heat radiation efficiency at the edge of the prismatic outer can 15 and thus reduce the likelihood of occurrence of a crack between the prismatic outer can 15 and the sealing plate 16. In this case, radiation of heat in the melted portion is directed downward as indicated by the arrow C because the heat conductivity of the prismatic outer can 15 is much larger than that of the air. The heat conducted downward as indicated by the arrow C raises the temperature of a region F indicated by crosshatching, and thus heat radiation in directions indicated by the arrows D and E is reduced. This reduces the likelihood that, in the prismatic outer can 15 and the sealing plate 16 as shown in FIG. 6, the melted portion is cooled and hardened from an outer part thereof, and causes the melted portion to be cooled slowly. Therefore, the likelihood that a crack occurs in the boundary between the prismatic outer can 15 and the sealing plate 16 becomes extremely low.
In the prismatic sealed battery as disclosed in JP-8-77983-A, the edge of the prismatic outer can 15 is cut off by the predetermined angle α to form the heat radiation eliminating portion 15′, and thus the likelihood of occurrence of a crack in the weld is reduced. However, the weld in the prismatic sealed battery does not have much higher strength than that in a prismatic sealed battery in which the edge of the prismatic outer can 15 is not cut off.
In order to find out causes of the weld not having a sufficient strength even when the edge of the prismatic outer can 15 is cut off as mentioned above, the inventor carried out a variety of experiments. Based on experiment results as well as the following facts:    (1) Generally, the sealing plate has a larger thickness than the fitting portion of the outer can; and    (2) The sealing plate is made of a material which is softer than the outer can and has excellent heat conductivity is used so as to facilitate treatment of a gas discharge valve or a negative terminal, it was found out that the sealing plate side more likely causes heat to escape therefrom and thus is less likely to be melted than the outer can side, and thus when there is a difference in degree of melting relative to the joint between the sealing plate side and the outer can side, no sufficient melting depth or sufficient joint strength is obtained.